Gamma echo in forward nuclear resonant scattering of synchrotron radiation in an inhomogeneous medium

The gamma-echo technique in nuclear forward scattering of synchrotron radiation has been suggested. This technique is based on abrupt 180 degrees remagnetization of the sample at t = tau which creates the gamma-echo with a maximum at t = 2 tau. For theoretical calculations the Maxwell-Bloch formalism modified for multilevel nuclear states has been employed. The numerical simulation for a medium of any 'optical' thickness as well as for the case with superparamagnetic relaxation has been carried out. It is shown that (1) successive 180 degrees remagnetization at t(i) = (2i + 1)tau generates the gamma-echo signals at t(ei) = 2(i +1)tau, (2) the peak of the gamma-echo for an 'optically' thick sample lies above the curve of resonant response for one of the same thickness with no inhomogeneity of hyperfine fields and (3) superparamagnetic relaxation attenuates the gamma-echo signal and changes its form.